Aluminum surfacing



United States Patent O pas ALUFYmlUM SURFACING Michael N. Marosi, Los Angeles, Calif.

No Drawing. Application July 5, 1956 Serial No. 5,863

14 Claims. ((Il. 148-624) This invention relates to an improved method of providing a coating on the surface of aluminum products as a protection against corrosive action, also to the formulated materials used in practicing the method of the invention, and to the end products of the method.

Aluminum, especially aluminum of a high degree of purity, has a natural resistance to corrosion due to the formation of its surface of a thin transparent coating of aluminum oxide immediately upon the exposure of the surface of an aluminum product to the air, because of the great affinity of aluminum for oxygen. Thus a freshly cut surface of metallic aluminum is immediately oxidized and covered with a thin transparent coating of aluminum oxide about 000000015 inch thick which largely protects it from further oxidation and reaction except from substances which can dissolve or mechanically break through the oxide. This natural oxide coating is neither strong enough nor impervious enough to prevent corrosion under more difiicult conditions such as long continued exposure to atmospheric moisture, or shorter exposure to sea water, or salt air over or near .the sea or subjection to frequent mechanical contacts.

When this oxide coating is formed on chemically pure aluminum, the aluminum will resist the action of corrosive agents, whether in the ambient air, in water, or in other materials which contact it :in its utilization in industry or the arts. However chemically pure aluminum has little commercial use. For example 99.96% pure aluminum has a tensile strength of only about 7000 p. s. i. (pounds per square inch) :while with the addition of a small amount of impurities of iron and silicon for instance, which are ordinarily present in 99 to 99.6% commercial aluminum, the tensile strength is doubled. Certain alloys of aluminum have tensile strengths as high as 100300 13. s. ,i. Aluminum ,having a small percentage of impurities, and alloys of larger percentages of other metals or elements, has other desirable properties, which are lacking in pure aluminum, but which are sought in industry and general usage,;such

properties as hardness and castability.

Now, while these commercial forms of aluminum and many of its useful alloys also acquire naturally formed aluminum oxide coatingsflhey are subjecttdmore rapid corrosion possibility due to galvanic action within the alloy or to entry of corrosive agents through breaks in the coating into the body of the impure or alloyed aluminum which is not as resistant to corrosion as is protective coating of-a more effective character than the naturally formed aluminum om'de.

According to the present invention, this natural oxide is 'utilized to form by a novel treatment with other materials, a coating of greater strength and higher resistance to further oxidation or destructive action by 2,836,526 Patented May 27, 1 958 corrosive agents, for long periods of time. The coating produced by this invention is also pleasing in appearance enhancing the beauty of aluminum with a satin-like sheen of a distinctive and novel character.

There are at present in use several methods of providing protective coatings on aluminum products. Anodizing, one of these methods, is an electrolytic process by which a protective coating is deposited on the surface of the aluminum in lieu of the natural oxide coating. A similar process with a similar result, alumiliting is a reverse electrolyzing process. There are disadvantages in'these processes. They are relatively expensive, are attended by danger to the operators, due to the high voltages used, and by subjecting the surfaces to electronic bombardment they greatly decrease the electrical conductivity of the aluminum which decreases the value of the aluminum as a material for conductors, such as are used as antennae for radio frequency equipment. The aluminum skin is harder after electrolysis, which may be a disadvantage in some utilizations of aluminum products.

in another process, the alodine process, which is a. chemical process, a phosphate compound is deposited'on the surface of the aluminum. In still another process termed irriditing a protective coating is formed by the deposit of a chemical compound.

Some of these known processes mentioned above are not entirely satisfactory in respect to the degree of protection which they afford. The standard test for resistance to corrosion is made by the application to the aluminum surface, of salt water spray of 20% salt content at a temperature of degrees F., and a'humidity of and other specified conditions. 'The resistance lifeof the treated aluminum is measured bythe duration of theelapsed time before the surface .is dulled andlater pitted by the salt spray. It is standard practice to measure the resistance life of the treated surface of aluminum as one year for each 24 hour period of application of the salt spray without evidence of corrosion. Under this .test the aluminum surface coatings produced .under the known methods, vary widely in resistance .life from 5 or less years to ten years, most of the known -methods producing coatings which hold up for greatly shorter periods than the maximum of ten years.

The process of this invention is basically diiierent from any of the above ,or of other known processes for rendering aluminum products resistant to corrosion. In all of theseknown processes the surfaces are first cleaned of its natural oxides and all other surface materials, such as dirt, grease and the like, and then chemicals of different kinds in the different-processes respectively-are deposited on the surfaces inlieu of tie removed natural aluminum oxideAl O in the process of this invention the natural oxide which :is ever present on the surfaces of aluminum products is at least partially left on the surface andthis ,oXide is at least partially converted into an aluminum compound which, in an extremely thin film, overlies and protectsunderlying aluminum and alsoany unconverted natural oxide film which may be present, after the chemicalreaction which creates this aluminum compound is completed. The aluminum compound thus formed ,in the practice of this invention, forms a non-porous even film, which not only protects against the action of corrosive agents, but also, to some extent, against abrasive wear, and gives to the aluminum product a pleasing, aesthetic appearance.

Thewprocessis alchemical d'pping process and may be consideredas.havingeight or more'steps, some of which are relatively unimportant, and are included to give a practical and cornplete presentation of the entire treatment given aluminum.

processupon the oxide film adhering to its surface.

, ther chemical changes.

(1) In the first step of the process, the aluminum stock material, whether aluminum plate, aluminum extrusion or other stock, or the aluminum fabricated product which is to be treated, is immersed or otherwise brought into contact'with a cleansing liquid for the removal of the superficial layer on the aluminum of dirt, oil and grease. This is preferably a non-ionic detergent in a weak solution, as

for instance /2 %,'which by its wetting and emulsifying bath is preferably at room temperature.

(2)' In the second step of the process the' aluminum is dipped in a bath of a solution of sodium hydroxide, of preferably about 5% strength, for a period of one to six minutes for example, and at a temperature in the entand now cleaned surface layer of aluminum oxide is at least partially converted into sodium aluminate and other oxidation products, which are thereby freed from the surface of the aluminum and made available for fur- If impurities are present in the oxide, as they usually are, in this step of the process the oxides and sodium aluminate become slightly discolored, and the surface of the metal usually shows a slight etching. (3) In the third step the aluminum product is sub- This I neighborhood of 135 degrees F. Here the naturally presjected to a cold water rinse in a tank or otherwise. If

the tank is used for this rinse it is provided with a steady supply of clean water at room temperature. This rinse removes the sodium hydroxide, stopping its action upon the aluminum surface. a

(4) In the fourth, and probably the most distinctive step of the process, the aluminum product is immersed in a bath which is a water solution of four soluble norfmally solid chemical compounds, each in granular or sulfonate, thelatter preferably diluted with suitable .inert material. The Recobright-IO solution is. a rapidly.

acting highly ionized bath. The wetting agent in the bath of this fourth step, increases the wetting action of the water on the metal surface, so that the other three chemicals can react more readily on the aluminum'and its surface oxide or other coating. This detergent is highly ionic, and assists in maintaining the bath at the desirable pH. 7 a

After one to three minutes in this Recobright-l0 bath at room temperature, an extremely thin, almost monomolecular film of a complex salt is deposited on the surface of thealuminum. This salt may be correctly termed aluminum sulfamo fluorocitrate. However the coating probably consists of several of these citrates with a slightly different structural molecular arrangement of the element ions, particularly the sodium and fluorine ions. Whatever the exact actual chemical composition of the coating, which may vary among other reasons because of the impurities present during the reaction which occurs during this fourth step of the process, it is-a layer of material which will stand up under 250 hours of salt spray, indicating a useful life of more than ten years, which has high reflective power (a quality of great utility when the aluminum is used in heaters or in lighting equipment).

It also gives theisurface of the aluminum product on which it'has been formed a beautiful satin-like sheen, greatly increasing the attractiveness of the metal. The coating is also possessed of high durability.

This aluminum sulfamo fluorocitrate which is probably in most cases the major ingredient of the surface coating created by the reaction taking place in the fourth step, is a chemical compound of a complex nature, which is not known to have ever before been produced in nature or in the laboratory. Nor hasit ever been known as a coating for aluminum either associated or unassociated with aluminum oxide.

Two possible structural formulae of these citrates are given here:

Possible structures of thesultamo fluorocitrate complex dry physical mixture. The solution of this mixture should be at a strength of approximately 1% to 2% depending somewhat on small variations in the formula of thedry This bath is preferably mix which becomes the solute. at'room temperature. Since the solution in this bath is acid in character and has preferably a pH acid factor of p 1.5 to 2.5, the tank for this bath is provided with an acid resistant lining. 7

Since both the dry mix which becomes the solute, and the water solution of the mix before it reacts with the surf face layer of the aluminum product, consist of more than one chemical compound, and may not therefore be given a single chemical name, this dry mix will, to avoid the repetitive use of a long single term worded to indicate 'all of its. ingredients, be referred to herein by its trade name of Recobright-IO, followed by suitable contextual words, such as mix, bath or solution. Recobright- 10 is a novel formulated composition of four separate chemical compounds. The four compounds are sulfamic acid, citric acid, sodium acid fluoride, and an anionic type wetting agent, as for example sodium alkyl aryl wlth aluminum (5) When removed from the tank of Recobright-IO This bath has a chemical reaction on the oxidation prod-.

ucts freed in the bath of the second step of the process and not converted to aluminum sulfamo fluorocitrate, the product of this reaction being removed from the aluminum products and dissolved in the solution.

(7) This sixth step of the process, i. e. the immersion in the bath of Recobright-D, is followed by a cold water .rinse like that described in steps N0. 3 and 5.

degrees which. causes a rapid drying of the coated.

( 8) Then follows a rinse in .hot water of -190 metal, eliminating the-creation of water spots on the dried surface.

While the materials used in the several steps of the process have been found to be very suitable for .the purposes for which they have been selected, a .number of other substitute materials may be used in the place of some of these materials mentioned above. For instance sodium acid fluoride may be replaced by potassium acid fluoride or by acid fluorides of others of the group of metals of sodium and potassiurn. The much greater expense and difficulty of securing these other chemicals makes the sodium acid fluoride preferable.

The Recobright-lO mixture of four chemical compounds mentioned above in describing the fourth step of the process is satisfactorily made according to the following formula:

Percent Sulfarnic acid 40 Citric acid 44 Operating strength 1% solution Sodium acid fluoride Sodium alkyl aryl sulfonate 40% active Percent Sulfamic acid '39 Citric acid 45 Sodium acid fluoride '13 1,22 3; f gg Sodium allryl aryl sulfonate 40% I active 3 Sulfamic acid 41 Citric acid 40 Sodium acid fluoride 17 ff i gfg f ggfi Sodium alkyl aryl sulfonate 40% active 2 Sulfamic acid 43 Citric acid 43 Sodium acid fluoride 12 225 2 22 Sodium alkyl aryl sulfonate 40% active 2 Sulfamic acid 41 Citric acid 41 0 Sodium acid fluoride l5 i gg gi Sodium alkyl aryl sulfonate 40% active 3 Sulfamic acid 45 Citric acid 39 Sodium acid fluoride 13 225 gi Sodium alkyl aryl sulfonate 60% active l A study of these formulae seems to show that when some one or more of the four ingredients are present in the mix in low percentages, the solution made from the mix must be stronger to compensate therefor. If the percentage of any one ingredient is considerably increased, the mix still is effective but the excess of that particular ingredient being unnecessary for, and therefor left over from the reaction, has no other effect than to add to the cost of the operation per unit of aluminum processed.

Aluminum alloys, experimental work has shown, may be successfully treated with the process of this invention provided that like in the alloy Duralumin not more than approximately 5% of copper or zinc is present in the alloy. Alclad, which is a sheet of alloy surfaced on each side by relatively purer aluminum, is made more resistant to corrosion if the aluminum sheathing is treated by the process of this invention. The pure aluminum, of course, is surfaced with the ever present very thin film of oxide and this oxide goes through the chemical changes lhed above, as one step follows another in practising the process.

Aluminum products treated by the process of this invention do not require lacquering, and are stain resistant. As an example of an advantage of this, window frames of aluminum which are installed in contact with lime mortar will not be stained or chemically affected, in any way, being protected by the coating produced by the process of this invention. Costly lacquering previously resorted to, and relied upon to prevent staining, is thus eliminated.

The cost of processing aluminum products according to this invention has been found to be about St) per square foot. The cost of every one of the known processes for the accomplishment of protection against corrosion is higher than this. The cost of the anodizing process is estimated to be 5 times as great, i. e. 25 per square foot.

If in the Recobright-IO or Recohright-D mixture one of the materials is present in a much larger percentage than specified above or in the appended claims, the invention is still being practised, provided that the excess quantity is inert as to its effect upon the reactions.

The expression aluminum products as used herein defines any metal stock article or manufactured metallic article in which the metal element aluminum predominates, and includes chemically pure aluminum, commercial aluminum, aluminum articles coated with'aluminum oxide and otherox'dation -products, and aluminum alloys subject to corrosion.

I claim:

1. The process of coating aluminum precincts with a thin layer of adhering material formed by the chemical action upon aluminum oxide of a formulated Water solution of sulfamic acid, citric acid, sodium acid fluoride and an anionic type wetting agent in the approximate effective ratio of l:1:.33:.05 respectively.

2. The process defined in claim 1, in whicr alumlnum product is coated with a natural aluminum oxide, and the process consists of converting this aluminum oxide to a material formed by the chemical action upon the said natural aluminum oxide of a formulated water solution of sulfamic acid, citric acid, sodium acid fluoride and an anionic type wetting agent in the approxi mate efiective ratio of 111233;.65 respectively.

3. The process defined in claim 2, in which the aluminum product is initially first cleaned of foreign substances adhering thereto and the aluminum oxide is then treated to free the oxides from adherence to the aluminum product, and thereafter the said freed oxides are treated with a solution formulated of water, sulfamic acid, citric acid, sodium acid fluoride and an anionic type Wetting agent in the approximate effective ratio of l:1:.33:.05 respectively to chemically react with the freed oxides and to form and deposit on the surface of the aluminum product a thin coating of the material resulting from such reaction.

4. The process defined in claim 3 in which the reagent for freeing the oxides is sodium hydroxide.

5. The process defined in claim 3 in which the treatment material is a solution in Water of a dry mix of the following approximate percentages, sulfamic acid 42%, citric acid 42%, sodium acid fluoride 14%, and sodium alkyl, aryl, sulfonate 2%.

" 6. The processdefined in claim 4 in which the solution ofthe mix is approximately at least 1%.

7. A process for coating aluminum products, having an aluminumoxide coating, with a corrosion resistant film, comprising: cleaning the'product with an anionic wetting agent; dipping it in sodium hydroxide; rinsing it 7 in water; dipping it in a water solution of sulfamic acid,

citric acid, sodium acid fluoride, and sodium .alkyl aryl sulfonate in the approximate effective ratio of 1:l:.33:.05 respectively; rinsing it in water; dipping it in a water solution of sodium dichromate, sodium acid fluoride and sulfuric acid in the approximate eflective ratio of 1.7:2z10 respectively; rinsing it in cold water; and rinsing it in hot water. a

8. The process defined in claim 7, in which: the said sodium hydroxide is in anapproximately 5% water solution at a temperature of 130-140 degrees, and the time of immersion is from one to six minutes; the percentages of sulfamic acid, citric acid, sodium acid fluoride, and sodium alkyl aryl sulfonate in said Water solution are approximately 42%, 42%, 14% and 2% respectively of the solid matter of the solution, and the initially solid matter is approximately 1% to 2% of the solution, and the immersion time is approximately between 1 and 3 minutes; in which the said water solution of sodium dichromate, sodium acid fluoride and sulturic'acid is approximately a 4% to 5% solution, and the said'three chemicals are in the proportion of 1.7 parts sodium dichromate to 2 parts sodium acid fluoride to 10 parts anionic type wetting agent is sodium alkyl aryl sulfonate. 11. A water solution of the mix defined in claim 10, of a strength of approximately one to two percent;

12. A formulated material for. use in the treatment 7 of aluminum products having a thin coating of aluminum oxidationtproducts consisting of a dry mix of the following chemical compounds in comminuted form, sulfamic acid, citric acid, sodium acid fluoride, and sodium alkyl aryl' 'sulfonate, there being in the mix a sufficient proportion of each material to make available for chemicai reaction from a water solute of the mix relative quantities ,by weight of'each material in the approximate ratio -of l: l:.33:.05 respectively.

' '13. An'aluminurn product in which: the body' of the product is commercial aluminum with impurities of not more than approximately two percent of the body coated on its exposed surfaces with a of which the major component is a material resulting from the chemical reaction of aluminum oxide with a formulated water solution of sulfamic' acid, citric acid, sodium acid fluoride, and an anionic type wetting agent'in :the V approximate eflective ratio of l:l:'.33:.()5, respectively 14. An aluminum product in whichz-the major com ponent of the body of thearticle is aluminum and the said body is coated on its exposed surfaces with an extremely thin film of Which the major component is a material resulting from the chemical reaction of aluminum oxide with a formulated Water solution of sulfamic acid, citric acid, sodium acid fluoride, and an anionic type wetting agent in the approximate effective ratio of 1:1 33;.05, respectively.

References Citedrin the 'file of this patent v V t v UNITED STATES PATENTS 2,012,697

Tanner et a1. Aug. 27, 1935 2,250,508 Thompson et al. July 29, 1941 2,681,873 Deniston June 22, 1954 

1. THE PROCESS OF COATING ALUMINUM PRODUCTS WITH A THIN LAYER OF ADHERING MATERIAL FORMED BY THE CHEMICAL ACTION UPON ALUMINUM OXIDE OF A FORMULATED WATER SOLUTION OF SULFAMIC ACID, CITRIC ACID, SODIUM ACID FLUORIDE AND AN ANIONIC TYPE WETTING AGENT IN THE APPROXIMATE EFFECTIVE RATIO OF 1:1:.33:.05 RESPECTTIVELY. 